Certain composting and water purification treatment operations involve aerating wastewater to neutralize pollutants and promote the growth of aerobic bacteria useful for such composting and purification treatment operations. Aeration introduces oxygen into the liquid and agitates the liquid. To introduce as much oxygen as possible into the liquid, it is desirable to introduce the oxygen as small discrete bubbles so as to increase the diffusion rate and oxygen transfer efficiency. Agitating the liquid facilitates increased gas diffusion and oxygen transfer because agitation increases both the number of discrete gas bubbles present at the point of injection and the flow rate of liquid through the area surrounding the point of injection.
Aeration devices are conventionally mounted on a shoreline embankment or a dock or within a treatment facility building. Such devices commonly include a motor drive unit or power head that is situated above the water line. A hollow drive or impeller shaft that also serves as a gas conduit extends angularly downward below the surface of the water.
A variety of conventional apparatuses have been used to aerate wastewater. Examples of such conventional apparatuses are described in U.S. Pat. No. 4,844,843, issued to Rajendren on Jul. 4, 1989 and entitled WASTEWATER AERATOR HAVING ROTATING COMPRESSION BLADES; and U.S. Pat. No. 5,851,443, issued to Rajendren on Dec. 22, 1998 and entitled AERATOR WITH DUAL PATH DISCHARGE. The disclosures of U.S. Pat. Nos. 4,844,843 and 5,851,443 are hereby incorporated by reference in their entirety. In some conventional aerators, shaft driven propellers and forced air conduits deliver ambient gas to the location of the propeller. In such aerators, a bearing rotatably mounts the shaft in a housing and facilitates rotation of the shaft. The propeller is positioned below the surface of the fluid body, and the propeller agitates the water at the air outlet from the air conduit to mix the ambient gas with the water. In this way, oxygen bubbles are introduced into the wastewater, which is agitated at the site of introduction of the oxygen bubbles.
During aeration, it is desirable to introduce a large number of oxygen bubbles into the wastewater. Further, it is desirable to agitate the mixture of wastewater and oxygen bubbles strongly to promote distribution of the oxygen bubbles throughout the wastewater, thereby aerating a large volume of wastewater. One way of increasing both the amount of oxygen introduced into the wastewater and the degree of agitation is to increase the power of the motor drive unit or power head.
While increasing the power of the motor drive unit or power head can accomplish both of these goals, certain structural issues may arise at higher power levels. With increased power delivered to the propeller, the propeller transfers a greater amount of power to the water and exerts a greater downward force against the water. As a result of this greater downward force exerted against the water, the water exerts a greater upward reactive force against the propeller. The upward reactive force tends to urge the propeller upward. Left unchecked, this force would cause the propeller to rise out of the water, bending the forced air conduit in the process.
As described above, a bearing rotatably mounts the shaft that drives the propeller within the housing. Some aerators, such as an aerator disclosed in U.S. Pat. No. 5,851,443, incorporate an upper bearing and a lower bearing to rotatably mount the shaft within the housing. These bearings prevent the propeller from rising out of the water when the aerator is operated at high power, e.g., above about 15 horsepower (hp). Rather than causing the propeller to rise, the upward reactive force generated when the propeller operates at high power is substantially transferred to the bearings. As a result, the bearings can be subjected to considerable stresses during high power operation. These stresses can lead to premature failure of the bearings. In particular, as the bearings are subjected to stress, they deteriorate and allow foreign material, such as sand and dirt, to enter the shaft. As the bearings continue to wear away, the fit between various components of the aerator loosens, and vibrations increase until the aerator fails.